Here is reply of nanopool support about problem:
Thanks for contacting us.
Please note that, there has been a bug in the recent SIA hardfork, which has made SIA difficulty inconstant.
It is currently quite high, therefore there are much less blocks found than usual.
According to the calculations the bug will be fixed in the middle of January.
For more info go directly to: https://github.com/NebulousLabs/Sia/releases
Since we are using the PPLNS system reward, your balance updates only after the pool solves a block.
Please, check our block distribution in the Stats menu.